Communication devices that perform wired or wireless information communications are formed with various high-frequency components such as amplifiers, mixers, and filters. Among those devices, bandpass filters (BPF) each have resonators arranged in a line, and allow only the signals of a particular frequency band to pass through the filters. To effectively use frequencies in today's communication systems, the filter characteristics should preferably be sharp shutoff characteristics, so that the available bandwidth can be used to a maximum extent. Further, in response to the demands for smaller communication devices, those filters should preferably be small in size.
To achieve desired filter characteristics, it is necessary to connect resonators to one another in an electromagnetic field. The circuit constant of the filter is formed with the resonant frequency fi of each resonator, the coupling coefficient Mij, and the external quality factor Qe with the outside circuit.
FIG. 14 is an equivalent circuit of a conventional bandpass filter circuit. In FIG. 14, reference numeral 901 indicates input terminals, reference numeral 902 indicates output terminals, reference numerals 903(1) through 903(n) indicate resonators, and reference numerals 904(1) through 904(n−1) indicate coupling circuits. This filter circuit is formed with the resonators 903(1) through 903(n) cascade-connected as shown in FIG. 14. The equivalent circuit of each of the resonators shown in FIG. 14 is formed with an inductor L and a capacitor C, and a resistor is added to the equivalent circuit if a loss effect is taken into account. The resonant frequency of a resonator without a resistor is expressed by the following equation:f0=1/sqrt(L*C)where L and C represent the inductance and capacitance of the resonator.
In the filter circuit of FIG. 14, the resonators are cascade-connected, and the frequency pass range and the stopband attenuation of the filter circuit can be determined by appropriately deciding the coupling coefficient Mij (m12, m23, . . . , mn−1, n in FIG. 14) expressing the coupling amount of each of the resonators, and the value of the external quality factor (Qe in FIG. 14) expressing the coupling amount between the resonators and the input or output circuits.
Since a current flows to the respective resonators in a filter having the resonators connected in a cascaded circuit, all frequency components in the current flow into the resonators. Therefore, in the case of the resonators are made of a material having a current capacity, such as a superconductor, a power handling capability of each resonator is an important parameter for allowing large power to pass through the filter circuit. Studies are being made to develop a technique for improving the power handling capability by taking measures to prevent concentration of current flow on the resonator by application of circular disk-like resonators or wide transmission lines.
In a superconductive resonator made of a superconductor, however, the un-loaded Q value is extremely high, and therefore, the current concentration in the resonators becomes larger. As is apparent from this fact, high power capability is difficult to realize by changing the shapes of the resonators.
FIG. 15 is a equivalent circuit of another conventional filter circuit. As shown in FIG. 15, the resonators are connected to parallel circuit, which the input power is dispersed each resonator in the filter circuit, the resonators 913(1) through 913(n) are connected in parallel so as to form the filter circuit (as disclosed for example in JP-A-2001-345601 (KOKAI) and JP-A-2004-96399 (KOKAI)). By such a parallel circuit, inputted power is divided to each of resonator 913(1) to 913(n) so as to increase the power handling capacity as a whole.
To connect the resonators in parallel, the respective resonators are designed to have different resonant frequencies from one another (f1, f2, . . . , fn in FIG. 15), and combining the resonators such that the resonators with adjacent resonant frequencies have mutually reversed-phase, to realize the filter characteristics. In FIG. 15, the symbol “-” of “-m2” indicates reversed-phase coupling. The structures of filter combining a superconductive filter and a normal conductive filter were disclosed for example in JP-3380165, JP-A-H11-186812 (KOKAI).
Although a superconductive filter and a normal conductive filter are connected in parallel in JP-3380165, the high power is inputted in both filters. When high power is supplied to the input, the power is divided and inputted into the filters, the divided power is separated only into power to be reflected in and power pass through each of filters, and with this shape, the superconductive filter also requires a high power handling capability. Furthermore, as a superconductive filter and a normal conductive filter are combined, the combined loss is increased, and it is impossible to take full advantage of the low loss effect of the superconductive component.